The battery has become a primary power source for many portable electronic devices such as radios, compact disc players, televisions, cameras, calculators, and the like. The development of these battery systems requires the compatibility of an electrolyte possessing desirable electrochemical properties with highly active anode materials, such as lithium, sodium, calcium and the like, and the efficient use of high energy density cathode materials such as iron sulfide-containing cathodes, metal oxide cathodes and the like. Under abuse conditions, such as forced charging, forced discharging or external shorting, these cell systems could be subject to leakage or in some instances even cell disassembly. To protect some cells against abuse conditions, the cells are generally equipped with a vent mechanism to permit the electrolyte to escape when the internal pressure exceeds a predetermined limit. Although a vent mechanism may protect the cell against disassembly, the electolyte that escapes may cause damage to the device being powered or any subject that it may contact. Consequently, in some cell systems, it is desirable to have safety means other than venting mechanisms that would disconnect the electrical circuit of the cell when subjected to abuse conditions and thereby prevent electrolyte leakage or cell disassembly.
In copending U.S. patent application Ser. No. 217,502 filed on Jul. 11, 1988 an electrochemical cell is disclosed that employs a current collector-safety switch member composed of an electrically conductive shape memory metal alloy in the electric circuit of the cell, a portion of which member retracts and breaks or disconnects the electrical circuit of the cell when the internal temperature of the cell exceeds a predetermined level. This results in terminating the temperature rise in the cell due to forced charging, forced discharging or external shorting.
In addition to employing safety means such as an internal circuit interrupter as disclosed above, the cell components must be selected so that once the circuit is disconnected, it remains disconnected. For example, when employing a conductive memory metal alloy as described above, the memory alloy, once activated to open the internal cell circuit, should remain fixed so that it does not close the internal cell circuit and thereby subject the cell to the same abuse condition that activated the memory metal alloy. However, in lithium cells, such as Li/FeS.sub.2 cells, intermittently opening and closing of the cell's internal circuit can be caused by lithium plating and the formation of lithium dendrites, particularly in the vicinity of the circuit interrupter means. Thus under adverse conditions, the circuit interrupter means can effectively be electrically bypassed by this intermittent reconnection of the internal cell circuit, often referred to as chattering. The dendrite formation could also produce contact to another conductive member electronically connected to the cathode which may result in internal cell shorting. This internal shorting could result in cell disassembly in some instances.
Many electrolytes have been recited in the art as suitable for use in cells, such as lithium cells. For example, U.S. Pat. No. 4,129,691 discloses an electrolyte for use in lithium primary cells which is made from a mixture of three organic solvents and an alkaline solute. The first solvent is chosen to have a dielectric constant equal to or greater than 35 (e.g. propylene carbonate), the second solvent is a linear polyether with its ether functional groups in the .gamma. position (e.g. 1,2-dimethoxyethane) and the third solvent has a high solvation power for dissolving large quantities of the alkaline salt (e.g. 1,3-dioxolane). The solvents are chosen that the conductivity maxima of the elctrolytes that would be obtained by mixing the solvents in pairs are higher than the conductivity maxima of the electrolytes that would be obtained by using each of the solvents on its own. Although 1,3-dioxolane has a high solvation power for dissolving large quantities of alkaline salt, in many cell systems it wouldn't provide a suitable conductivity for the electrolyte solution at a molar concentration of the solute of 1 molar per liter or less.
Japanese Patent Application 57-50772 discloses an organic electrolyte for cells which comprises propylene carbonate, dimethoxyethane and dioxolane and wherein the volume of dioxolane in the mixture is kept at between 5% to 10%.
Japanese Patent Application 63-152886 discloses a nonaqueous secondary battery employing an electrolyte which comprises a solvent of 1,3-dioxane or a solvent mixture containing 1,3-dioxane, an anode of lithium or lithium alloy and a cathode of a manganese dioxide-base material. The 1,3-dioxane solvent is stated as forming a protective film on the anode through reduction of the solvent which suppresses the formation of lithium dendrite growth that usually occurs during the discharge-charge cycling of the battery.
One object of the present invention is to provide an electrolyte solution suitable for electrochemical cells that will effectively prevent internal shorting of such cells when subjected to certain kinds of abusive conditions.
Another object of the present invention is to provide an electrolyte solution admirably suitable for cells employing internal circuit interrupter mechanisms.
Another object of the present invention is to provide an electrolyte solution admirably suitable for lithium cells, particularly lithium cells employing a coiled rolled electrode assembly.
Another object of the present invention is to provide an electrolyte solution ideally suited for cells employing a lithium anode and an iron sulfide-containing cathode.
The foregoing and additional objects will become more fully apparent from the following description.